1. Field of the Invention
This invention relates to a process for preparing polymeric beads
2. Description of the prior art
Cross-linked hydrophilic polymers and their use in contact lenses and other biomedical devices is well known For some applications it is preferable to use these polymers in a particulate form rather than as a cast or die cut article Particulate polymer can be obtained by grinding previously prepared polymers or by employing certain processing techniques during suspension polymerization
The conventional suspension polymerization process can be controlled readily to produce globular particles 0.1 to 2 millimeters in diameter With the use of high-speed stirrers and emulsifiers, the size of the individual particles can be reduced to 20-100 microns When smaller particles are needed, coarser material is crushed or ground, and the comminuted material is classified according to size, hydraulic classification usually being necessary for fractions smaller than 100 microns.
U.S. Pat. No. 3,583,957, R. C. Chromecek et al, discloses that when monomers having one olefinic bond, e.g. ethylene glycol methacrylate, and two olefinic double bonds, e.g. ethylene glycol dimethacrylate, are copolymerized under quiescent conditions to obtain globular particles smaller than ten microns in diameter when at least 40 percent of the polymerization mixture is a solvent inert to the monomers and to the resulting copolymer The monomers are initially dissolved in the solvent whereas the copolymer is insoluble in the solvent and is not significantly swelled by the same. Under these conditions, a mass of loosely cohering globular copolymer particles of the desired size is formed. The mass is readily disintegrated into the individual particles by applied light pressure or by simply immersing the mass in an inert liquid This process, however, is not efficient for continuous or large volume production, requires high temperatures and because of the temperature employed tends to fuse the particles produced
Hydrophilic polymers have been closely associated with the contact lens and biomedical device fields Typically an elevated temperature has been employed to prepare these polymers, see U.S. Pat. No. 3,532,679, R. Steckler, which discloses temperatures of 35.degree.-60.degree. C. U.S. Pat. No. 3,700,761 F. O'Driscoll et al, which discloses a first stage temperature of 40.degree.-60.degree. C. and 90.degree.-120.degree. C. for the second stage, and U.S Pat. No. 3,822,089, 0. Wichterle, which discloses temperatures of 60.degree.-85.degree. C.